This invention relates to position transducers and more particularly, to a miniaturized incremental encoder with low power consumption.
An incremental encoder is used to measure the angular displacement of a rotatable shaft. Typically, the incremental encoder comprises a disc having circumferentially arranged slots mounted on the shaft between a plurality of light source-sensors pairs. Each time a slot in the disc passes a light source-sensor pair, the sensor generates an electrical pulse. In the prior art, it is common to space two of the light source-sensor pairs circumferentially such that the sensor outputs are in quadrature phase relationship. The combination of the outputs from these sensors provides four counts as the disc is displaced by an angle equal to the slot spacing. Common practice is to space the two quadrature phase generating light source-sensor pairs apart by an angle greater than the slot spacing. The phasing of the sensor outputs relative to each other represents the direction of the angular displacement and the change of states of the sensor outputs represents the incremental magnitude of the angular displacement. A bi-directional counter is operated responsive to the outputs of the sensors so as to accumulate the displacement representation, taking into account direction, and thereby provide a representation of angular position. It is common to employ a third light source-sensor pair to generate a marker signal that resets the counter and performs other control functions.
Manufacturing tolerances result in a variation of the radial distance from the axis of rotation of the disc to the quadrature phase light source-sensor pairs as the disc rotates. This variation gives rise to a change in phasing between the sensor outputs in direct relationship to the circumferential spacing between the light source-sensor pairs.
In some applications of an incremental encoder, the slots are placed on only part of the circumference of the disc. In such case, the counter does not operate correctly near the ends of a group of slots because one quadrature phase sensor starts to operate a number of counts before the other.